Dern was born in Phoenix, Arizona, the daughter of Wellington "Megaton" Dias, a highly decorated grappling competitor himself.
Mackenzie began training by 3 years old and practicing with her father and stepmother who is also a black belt, Luciana Tavares. Mackenzie started competing at a young age, and began competing in the adult divisions by 14 years old. She has won a world championship in every belt level as she progressed to black belt rank, which was awarded to her by her father in December 2012, three months prior to her 20th birthday. She graduated from Ironwood High School in Glendale, Arizona.